*So, since each boss level 21+ has a .01% to drop ONE of the above items, and there are 5 bosses (including the rare-spawn boss), there is a .05% chance that ONE of the above items drops during a run. *

Actually, that's wrong. To properly calculate the probability of an event occurring in a given number of tries, you have to take the chance that it doesn't occur each time and multiply them, then subtract that value from 1. So in actuality the chance to get an item is 1-(0.999^5)=0.00499.

Granted, in this example, it really doesn't matter because we're dealing with tiny numbers anyway. But it's a common fallacy in dealing with probability. A 50% chance doesn't mean you have a 100% chance after two tries, it means you have a 75% chance. But where this does come into play in this case is here:

*(.05%) * (20%) = .01% (roughly 1 Shadowfang every 10000 runs!!!)*

You are not in any way guaranteed to get a Shadowfang after 10000 runs. In fact, 10000 runs is defined probability-wise as the number of attempts it takes for you to have had a 60% chance to get it. Almost half of the people who make 10000 runs still won't have it. For it to properly average out to 1 every 10000 runs, you'd have to take considerably more samples. It'd be much more reasonable to expect to have 100 after a million runs, or a thousand after ten million. but even then your numbers are so relatively small that statistical error will skew the values a bit.

The general point is the same, though: Don't farm Shadowfang. Buy it if you want to twink, go level if you want it for a character you're leveling.

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